The prior art includes a center cutting end mill as disclosed in U.S. Pat. No. 3,938,231 wherein square inserts are positioned with zero axial lead angle for axial plunge cutting or for radial feed; also indexable insert drills of the type shown in U.S. Pat. Nos. 3,540,323 and 3,963,365 which employ a pair of square inserts each having eight indexable cutting edges positioned so that the active lead cutting edge of one cuts the inner half while the other cuts the outer half of the hole.
A further development shown in U.S. Pat. No. 4,124,328 provides an improved construction capable of use in drilling hole depths as great as 3 to 31/2 times drill diameter which are clean, smooth and straight without relying on wear strips or shank surface contact with a hole wall for guidance. A pair of inserts are employed having only slight but identical lead angles in the range of 0.degree. to 5.degree., which may be either positive or negative, the cutting edges of which sweep through a common cone of revolution with one radially innermost insert having an arcuate corner intersected by the drill axis and the other radially overlapping outermost insert having an outer arcuate corner establishing the cutting periphery and the diameter of the drilled hole.
Other prior drills are known having a replaceable insert with an uninterrupted cutting edge extending above a radial line and terminating without extending overcenter.